Date: Wed, 6 Nov 2002 11:22:23 +0200 From: Ruslan Ermilov <ru@freebsd.org> To: Joerg Wunsch <joerg_wunsch@uriah.heep.sax.de> Cc: freebsd-scsi@freebsd.org Subject: Re: ``mt erase 0'' on a non-rewinded tape Message-ID: <20021106092223.GB93420@sunbay.com> In-Reply-To: <200211051918.gA5JIqaY014094@uriah.heep.sax.de> References: <20021105122423.GA79188@sunbay.com> <200211051918.gA5JIqaY014094@uriah.heep.sax.de>
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On Tue, Nov 05, 2002 at 08:18:52PM +0100, Joerg Wunsch wrote:
> Ruslan Ermilov <ru@FreeBSD.ORG> wrote:
>
> > The script always used to fail with EINVAL attempting to run a
> > quick erase, ``mt erase 0''. After a bit of experimenting, it
> > turned out that `erase' only works if I rewind the tape (either
> > through by using the rewind device, or by running the `rewind'
> > or `retension' commands in advance).
>
> Please read the SCSI command description for your drive. Depending on
> the media type, the ERASE command will only be accepted at either
> beginning of medium, or at the beginning of partition for a
> partitioned medium. AFAICT, the FreeBSD driver doesn't support
> partitions anyway, so in effect, the drive will only accept the
> command at BOM.
>
OK, thanks for the explanation!
> Apart from that, i don't understand your reasoning for the erase
> command. Since the quick erase is a logical erase command only
> anyway, the effect is the same like starting to write from BOT, for
> any practical purpose.
>
My tapes always look like this:
level 0 backups at Friday's evening, level 1 backups on Monday, ...,
level 4 backups on Thursday. If I don't erase the tape when I do a
level 0 backup, won't there be a chance that when I later do an
incremental backup and do an "mt eom" it will find some stale file
markers left by old incremental backups?
> Whether you use the rewind or non-rewind device should make no
> difference, as long as the tape itself is at BOM. "non-rewind"
> actually means "do not rewind upon close of the device", while the
> state at open time is always the state the device has been left over
> by the last operation.
>
I wasn't clear enough. I meant it worked if I always used the rewind
device. In this case, "mt erase" was always run at the BOM.
> It's always good practice to keep the medium at BOM while the tape is
> not in use, since after all, that's the only position you can rely of.
> Otherwise, if a SCSI bus reset hit while your script was idle, the
> drive will rewind the tape, and next time your script is run it might
> make an invalid assumption about the actual tape position, thus
> perhaps accidentally overwriting something. OK, you wrote that you're
> using a "mt eom" before trying to append, so you're already safe
> there, but then there's no reason to not rewind the medium when
> finishing the script.
>
Yes, the script runs "mt eom" for that very reason. But why would I
rewind the tape if the next day I want to use it from this same
position? IOW, if the SCSI bus isn't reset in-between, I get:
# /usr/bin/time mt eom
0,01 real 0,00 user 0,00 sys
(I change the tapes once a week, at Friday's evening.)
Cheers,
--
Ruslan Ermilov Sysadmin and DBA,
ru@sunbay.com Sunbay Software AG,
ru@FreeBSD.org FreeBSD committer,
+380.652.512.251 Simferopol, Ukraine
http://www.FreeBSD.org The Power To Serve
http://www.oracle.com Enabling The Information Age
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